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A new barrier designed by a University of Waikato
researcher may prove that removing pest fish from lakes and
waterways can improve water quality, without forcing farmers
to minimise profits.

Waikato University researcher Dr Adam
Daniel says it’s well known pest fish such as koi carp
have played a key role in the a collapse of aquatic plants,
loss of native biodiversity, algal blooms and reduced
waterfowl production and that by removing them water quality
may improve.

Nutrient run-off from farms has created an
ideal habitat for pest fish to thrive within the Waikato
region but University of Waikato researchers are hopeful
removing pest fish can help restore some lakes without
forcing farmers to drastically reduce their fertiliser
use.

Dr Daniel has designed and installed a barrier on an
outlet stream coming from Lake Ohinewai – near Huntly.
“We know from previous research conducted by the
University of Waikato that fish frequently move out of lakes
and then return so we are taking advantage of that behaviour
by installing a one-way barrier.”

By removing the
remaining koi carp population and measuring the resulting
water quality, scientists hope to prove pest fish are a
major cause of damage to Waikato lakes and the river and
provide a template for cleaning the lakes up.

While
researchers are hopeful that pest fish removal alone will
improve water quality it is likely a combination of nutrient
reduction and fish removal will be necessary to restore the
Waikato’s shallow lakes.

The barrier is designed to
allow koi carp to move on to Lake Waikare and the Waikato
River but not return and is similar to a koi carp trap,
designed by Leigh Thwaites of the SARDI laboratory
(Australia), that will be installed next year at Lake
Waikare by the Waikato Regional Council.

“It’s a real
simple system really,” says Dr Daniel. These metal
‘fingers’ hang in the water and are weighted so the koi
can push through, but they only open one way so once the
carp have pushed through they’re locked out of the
lake,” said Dr Daniel who’s part of LERNZ, the Lakes
Ecosystem Renewal New Zealand programme based at Waikato
University and funded by the Foundation for Research Science
and Technology.

Waikato University biologists are at the
heart of a $10 million project over 10 years to research and
save New Zealand’s lakes. The programme focuses on the
Rotorua Lakes but will have a nationwide application.

A
study by Dr Daniel in 2008 showed carp travel between lakes
and the river using storm drains and streams and can travel
as far north as 800m from the sea at Port Waikato and as far
south as Otorohanga. There is a real danger pest fish will
continue to spread.

Lake Ohinewai is the perfect place to
test the barrier as it has about 12 inlets and only one
outlet, which allows carp to use the stream to seek out
other water bodies, while stopping them from
returning.

Koi carp damage lakes by eating the insects and
plant material found in the lake bed that native fish and
waterfowl usually would. In the process of doing this they
stir up sediment in the water causing algae blooms and
dirtying the water so plants can’t grow.

Earlier this
year University of Waikato scientists removed about 60% of
the koi from Lake Ohinewai. To date more than two tonnes of
carp have been removed from the lake, with more than 300kg
of fish caught in one catch. The Lake Ohinewai restoration
project was partly funded by the Department of Conservation
which played a key role in the planning and implementation
of the fish
removal.